The width of three millimetres is about one-tenth of an inch. The width of a penny is just over 1.5 millimetres, so the width of two pennies is about 3 millimetres.
In the metric system, a millimetre is a unit of measurement that is equal to one thousandth of a metre. The millimetre is the smallest measurement you can see on a metre stick. In the metric system, one inch is equal to 25.4 millimetres. The width of a coin is measured in millimetres at the United States Mint. The width of a dollar coin is 2 millimetres, the width of a dime is about 1 1/3 millimetres, and the width of a nickel is almost 2 millimetres.